U.S. Pat. No. Re. 29,401 to Aker et al. discloses a speed measurement or radar system for determining the speed of a vehicle while the platform for supporting the radar is also moving. In this regard, two different Doppler signals are generated. A first Doppler signal is indicative of the speed of the radar platform. A second Doppler signal is indicative of the relative speed between the approaching vehicle and the radar platform. The radar includes an automatic gain control (AGC) circuit which receives each Doppler signal after it has been amplified. The Doppler signals are then processed to provide a digital read out representative of the speed of the approaching vehicle. U.S. Pat. No. 3,721,977 to Darboven describes an aircraft velocity sensing system which uses four switchable beams and provides an indication that a lock-on of an image beam has occurred. The system includes a detector of outputting a Doppler frequency spectrum related to the velocity component of the aircraft. The Doppler signals of each of the beams are passed through a band pass filter and then through an AGC to control the level or amplitude of the Doppler signals in each returned beam. The level of Doppler signals outputted by the AGC is controlled by frequency tracking loop. The frequency tracking loop includes a discriminator having a cosine mixer and low pass filters. The cosine mixer signal is fed back to the AGC so that the AGC is controlled by the level of the center frequency tracked by the frequency trackin loop. The frequency tracking loop also includes a voltage controlled local oscillator which is maintained at a frequency proportional to the center frequency in the Doppler spectrum. This frequency output signal is applied to a velocity computer for determining velocity of the aircraft. British Pat. No. 1,468,159 to Whetton et al. describes a speed measuring system for use with vehicles wherein a broad spectrum of Doppler frequencies is first applied to a high pass filter. The cut-off frequency of the high pass filter is adjustable by a signal fed back to the filter. The cut-off frequency corresponds to the frequency of the signal having the maximum amplitude which is detected in the Doppler spectrum inputted to the high pass filter.
The present invention is directed to an apparatus for minimizing harmonic distortion in a microwave circuit which outputs a number of Doppler signals at substantially the same time. The multiple Dopper signals are generated as a result of measuring simultaneously, or very close in time, the velocity of two or more objects. If at least one Doppler frequency is harmonically distorted at the output of the microwave circuit, subsequent processing of the Doppler signals to provide a velocity measurement may be falsified or impeded by the presence of unwanted harmonics of the fundamental Doppler frequencies.